On the morning of April 15, 1945, 70 years ago this month, a convoy of Canadian and British soldiers liberated Franci Solar, a 25-year-old Czechoslovakian dressmaker, from the Nazi concentration camp at Bergen-Belsen, in northern Germany. She had been living in camps, including Theresienstadt and Auschwitz, since 1942.

She was undernourished and puffy and had typhus. When she finally made her way back to Prague, weeks later, friends were surprised to find her alive, which was why they had sold or assumed possession of her belongings. The odds were in their favour. Of the nine million Jews who had lived in Europe before the war, and of the perhaps 500,000 who survived in Nazi-occupied Europe in concentration lager, or fighting in small bands of partisans, or hiding, or passing as Christians, no more than 75,000 outlived the camps.

Franci resumed a mostly solitary life in Prague – her parents and husband had been killed by the Nazis – until she married Kurt Epstein, a 41-year-old former Olympic water-polo player and fellow camp survivor, and in 1947 gave birth to their daughter, Helen – Helen, after Kurt's mother, also killed. After the Soviet-backed Communist coup in Czechoslovakia in 1948, the Epsteins moved to Manhattan. About 92,000 Holocaust survivors emigrated to the United States. An additional 25,000 headed to Canada.

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Thirty-one years later, Helen published a book. It was a story no one had ever told before, at least not in a book meant to be read by everyone.

Children of the Holocaust: Conversations with Sons and Daughters of Survivors – published to widespread acclaim in 1979, 35 years ago last fall – made a then-astounding claim: that the harrowing trauma of the Holocaust, and the symptoms that marked survivors, had been passed on to their children – a generation that wasn't even alive during the war.

Today, having identified post-traumatic stress disorder, we take this as a given. And the transmission isn't just psychological, as psychiatrists have pointed out for more than a century; it's physical. Rachel Yehuda, a pioneering psychiatrist and neuroscientist at Mount Sinai School of Medicine in New York, has found that children of mothers with PTSD are three times as likely to have it as are other kids, and almost four times more likely to be depressed and anxious. As Judith Shulevitz pointed out last fall in The New Republic, the children of survivors often have unusual levels of cortisol, a hormone released in response to stress – which was also true of infants whose mothers were pregnant and near the World Trade Center on 9/11.

Emotional and physical trauma can become genetic.

I reread Helen Epstein's book a few days ago, as we came up on Easter and Passover, the annual celebrations of forgiveness and freedom and resurrection and renewal that take place this weekend. Reading it, you can't help but think of all the agony that has been stored up and passed on – by survivors not just of the Holocaust, but of Hiroshima and of Cambodia and of Rwanda, of residential schools and wars and countless other public and private calamities.

But the more we learn about how deep and far a wound can travel, the more we seem to want to forget it. The world is plagued by genocide (Islamic State, Boko Haram), ethnic wars (Ukraine, Mali, Burma), physical, sexual and racial abuse (disappearing aboriginal women, men's fraternities in Oklahoma). It made me want to remember what Helen Epstein noticed 35 years ago, and how the world responded. Because 35 years from now, another generation's scars will be rippling to the surface.

Helen Epstein was a student at Israel's Hebrew University in the late 1960s (she's 67 now) when she first felt a connection to other children of Holocaust survivors – multilingual citizens of a global array of countries whose parents were all Central European survivors of concentration camps.

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"I wasn't even thinking about trauma then," Ms. Epstein remembers. "I was just thinking, 'Here are all these people who speak European languages like me.' And I was connecting with them." Never before – not at school in New York, not playing music (she is a widely published cultural journalist and biographer of, among others, Vladimir Horowitz and producer Joe Papp), not in the Brownie Scouts – had she ever felt part of a community.

Her sense of isolation was itself a symptom. Until then, the Holocaust had been what Helen believed was her private secret, a "black box" within her where she stored the details of her parents' life in the camps, and her own vivid, almost hallucinogenic mental images: piles of skeletons and hills of suitcases, barbed wire – none of which she had seen with her own eyes. She lived within a "floating sense of danger and incipient harm." Manhattan's Seventh Avenue subway line was a cattle car on the way to Poland. A smokestack was always attached to an imagined crematorium. She often felt angry and violent, but had nowhere to park those feelings.

In hundreds of interviews over the next decade, including a two-year stint of research in Toronto, she discovered how common and debilitating these fears were. They recurred again and again in the lives of children of survivors.

The parents of one of her closest friends never left their house, for fear it would be looted or burnt down: This seemed like a reasonable concern to Ms. Epstein. Many children of survivors married other children of survivors: Raising a family took on "cosmic significance." Some hid their Jewishness; many never talked of the war, "because talk meant accepting that the war had happened and, more than anything else in the world, I wished it had not," as one subject explained. Many were named for murdered relatives. Many were enormously accomplished people, but they still felt like replacements, with all the burden that implied. Hardly anyone discussed these matters.

Most of all, they had to deal with their parents, who had survived unthinkable Nazi torture to give their children a life free from it. "For most kids of survivors, there's nothing except your parents," Ms. Epstein told me recently from Lexington, Mass., where she writes and lives with her husband, Patrick Mehr, an e-book publisher and himself the son of French survivors, "There's no grandparents. No aunts or cousins. What does that mean? It means, first of all, that your parents have no context. There's no one to say, 'No, your mother didn't do that, your father's wrong.' It essentially magnifies the power of the parents."

Quite apart from hearing harrowing stories of murdered relatives – proof, as Ms. Epstein says, "that something went really wrong in your family"– the children of survivors had no allies. The ensuing pressure to cause no trouble, to outperform others, to make up for all the losses of the war, was overwhelming. Some had trouble settling on a career; many figured any choice would fail; many were anxious. "I often wonder if I could have survived myself and I doubt whether I could have," one interviewee tells the author. Says another: "We had to be gentle with our parents. They appeared to be very strong people but we had to be gentle with them because they could shatter very easily."

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Every time Albert Singerman, one of Ms. Epstein's subjects, disobeyed his mother, she screamed "Enemy of Israel! Enemy of the Jews!" He responded by trying to see how long he could hold his breath in shower stalls, and later enlisted and fought in Vietnam.

Ms. Epstein herself often felt numb, as if she had no right to be angry. Her solution was to dissociate from what was happening. She longed to hear her family's stories, but she was loath to cause her parents any pain in the telling. Everything took place under a curtain of secrecy – and in a mixture of Czech and German, to boot. "I heard all this in a language that wasn't my adult language"– loaded words like lager and kapo and achtung, which didn't seem to have any precise English equivalent. "It made it feel like a different world," Ms. Epstein says today. "And maybe that speaks to why no one knew. It was like another planet."

Thirty years went by before these problems were identified in the children of survivors. Ms. Epstein estimated there were at least 250,000 children of survivors like her. Today, there are, conservatively speaking, somewhere between 10 million and 52 million refugees on Earth, people who have been ripped out of their lives and families for the crime of being who they are. Presumably their offspring will have a few issues themselves.

For all the corroboration Ms. Epstein found, there was very little acknowledgment of the syndrome scientifically. By 1978, fewer than two dozen studies of children of survivors had been published. The first, a casual description of three patients, was written in 1966 by Vivian Rakoff, the brilliant professor emeritus of psychiatry at the University of Toronto (and later director of what became the Centre for Addiction and Mental Health), who at the time was an assistant director of research at the Jewish General Hospital in Montreal, another gathering city for survivors.

"The parents are not broken conspicuously," Rakoff wrote, "yet their children, all of whom were born after the Holocaust, display severe psychiatric symptomatology. It would almost be easier to believe that they, rather than their parents, had suffered the corrupting, searing hell." Two of his three subjects had tried to commit suicide.

Twelve years later, Rakoff and two colleagues published the first systematic study of children of survivors. He was violently criticized for pathologizing them, and while he now points out that "the vast majority of the kids of survivors do okay," he sees nothing surprising in his original thesis. "It's not surprising that a tremendously traumatic event would be imparted to successive generations. Human beings are shaped by experience. And experience becomes part of memory. And memory is what we impart to our children." (Dr. Yehuda's theory, that stress reactions caused by severe trauma are genetically inheritable, has run into skepticism as well.)

That changed as Helen Epstein's book appeared, first in the form of a New York Times Magazine story. Some survivors hated it. "It really upset Holocaust survivors who were invested in normal children, and in not giving Hitler a posthumous victory." But the 500 letters that arrived in response – detailed seven- and eight-page single-spaced accounts of similar experiences – proved her hunch had been right.

Children of the Holocaust has been in print ever since. Germany, Italy and Japan, three countries of the Axis, were the first to translate it outside the United States. It still hasn't been published in Israel.

"Israel has always been extremely ambivalent about the Holocaust," Ms. Epstein said, over the telephone. "In 1967, when I was starting my inquiry, Israel was 19 years old, exactly as old as I was. So it was a very new country, and they were really trying to differentiate between Israelis and Diaspora Jews." Israel was trying to re-establish a stronger, healthier, prouder, fitter image of Jewishness.

As well, Ms. Epstein added, "Everyone in Israel is some kind of survivor," whether of 19th-century pogroms in Russia or of more recent Jewish exoduses from North Africa and the Middle East. "So you have a country that's totally populated with people who have PTSD. Why would they be more interested in my parents' history than they would be in their own? Plus, my family's story doesn't fit the Zionist narrative, because my parents emigrated to America."

These days, with her husband, she runs Plunkett Lake Press, a non-fiction publishing company with a focus on Jewish writing (including Heda Kovaly's Under A Cruel Star, one of the most gripping Holocaust memoirs ever published, which Ms. Epstein discovered and translated.) Plunkett Lake was the spot in the Berkshires where Kurt Epstein liked to swim after he came to America.

Ms. Epstein has two grown sons. They grew up with relatives, so "they're not as preoccupied by history." The transmission of historical trauma from generation to generation can be halted, it seems, provided one works to becomes conscious of it.

But history keeps grasping for us anyway. Ms. Epstein is now finishing the last in what has turned out to be a trilogy of memoirs about the long and often invisible hand of the Holocaust in her life: All she will say is that it uncovers sexual abuse, an affair her mother had, and our astonishing ability to "forget" what we most need to remember.

Trauma is trauma, whether it is besetting children of Holocaust survivors or children of families shattered by atom bombs, civil war, terrorism, domestic violence, sexual abuse, addiction, or even illness and disability. The stories keep emerging: in Heather Connell's Small Voices, a film about the children of survivors of the Khmer Rouge killing fields; in Peter Balakian's memoir Black Dog of Fate, written as the son of survivors of the Armenian genocide; in Michael Arlen's Passage to Ararat, also about Armenia. (Memoirs by the children of Rwandan survivors are rarer: They're just becoming adults.) The details of each oppression make it unique, but the effect of the trauma always follows the same path.

"I don't feel possessive about my PTSD at all," Ms. Epstein says. "I think it's nearly universal." To which Vivian Rakoff adds, "I think the transmission of trauma has to be admitted to. That when you do something terrible, it has effects. You can have psychic transmission of disorder in the same way you can have microbial transmission of disorder."

Now we are learning that the horror can be passed along physically, and perhaps even genetically. Efforts are being made to interrupt that fateful flow: At Mount Sinai in New York, Dr. Yehuda has a theory that hydrocortisone might stymie the establishment of PTSD. There are also encouraging therapies and experimental programs, as Judith Shulevitz reported in The Atlantic, in which pregnant women at risk for PTSD receive counselling to help them through the thickets of child rearing.

Trauma and the atrocities that cause it are unavoidable. Parliament's decision to expand Canada's war against the Islamic State is, at least arguably, a legitimate and necessary evil. But the children of the soldiers and victims who fall on both sides in that war will feel its trauma regardless, in some place too dark to see. Then will come the hard part. Because once we notice trauma, and inquire after it, we are apologizing for it, and admitting to some sense of responsibility.

Maybe this is why we try so hard not to to notice other people's pain, why we resist the idea that formative experiences are passed along in physical form as memory, conscious or collective or otherwise. We know we're connected to one another in ways we can't see or control, inconvenient as the fact often is. "Much of history is written in blood," Helen Epstein writes in Children of the Holocaust, "and experiencing some degree of trauma seems to be a part of experiencing life. What that means to me is that it is not 'other' but, to various degrees, 'us' and that we need to learn to use that insight toward connection rather than separation." Human pain turns out to be not very private after all.

Judith Herman, the Harvard psychiatrist who in 1993 wrote Trauma and Recovery: The Aftermath of Violence from Domestic Abuse to Political Terror, observed that "the study of psychological trauma has a curious history – one of episodic amnesia." Why? Because "to study psychological trauma is to come face to face both with human vulnerability in the natural world and with the capacity for evil in human nature."

We want to remember, and we want to forget. We are who we are. But sometimes we can't bear to admit it.